1. Field of the Invention
The present invention relates to a rechargeable lithium battery which includes a lithium-aluminum-manganese alloy negative electrode containing lithium as active material, a positive electrode and a nonaqueous liquid electrolyte.
2. Description of Related Art
It is known that when metallic lithium is used for a negative electrode of a rechargeable lithium battery, the lithium deposited on charge tend to grow into dendrites which eventually hinder repetitive charge-discharge cycling of the battery. This has led to the study to use a lithium-aluminum alloy for a negative electrode of a rechargeable lithium battery. The use of lithium-aluminum alloy appeared to permit repetitive charge-discharge cycling of the battery since it is capable of electrochemical storage and release of lithium and thus unsusceptible to dendrite formation.
The lithium-aluminum alloy, when used for the battery negative electrode, is however subjected to subdivision as a result of repetitive expansion and shrinkage during charge-discharge cycles. This structural destruction results in the failure to obtain satisfactory charge-discharge cycle performance. In order to prevent such subdivision of the lithium-aluminum alloy during charge-discharge cycles, Japanese Patent Laying-Open No. Hei 9-320634 proposes the use of a lithium-aluminum-manganese alloy. This lithium-aluminum-manganese alloy provides a satisfactory charge-discharge cycle performance and has been found feasible as the negative electrode of rechargeable lithium battery.
However, as technology continues to push up performance and reliability levels of equipments, rechargeable lithium batteries using such a lithium-aluminum-manganese alloy for a negative electrode have come to show insufficient charge-discharge cycle performance characteristics, which has been a problem.
The present invention relates to improvement of such a rechargeable lithium battery including a lithium-aluminum-manganese alloy negative electrode, and its object is to provide a rechargeable lithium battery which exhibits good charge-discharge performance characteristics based on the improved nonaqueous liquid electrolyte.
In order to attain the above-described object, a rechargeable lithium battery in accordance with the present invention includes a lithium-aluminum-manganese negative electrode containing lithium as active material, a positive electrode and a nonaqueous liquid electrolyte containing a solute and a solvent. Characteristically, the nonaqueous liquid electrolyte further contains at lease one additive selected from trialkyl phosphite, trialkyl phosphate, trialkyl borate, dialkyl sulfate and dialkyl sulfite.
In the present invention, the at lease one additive selected from trialkyl phosphite, trialkyl phosphate, trialkyl borate, dialkyl sulfate and dialkyl sulfite may be incorporated in the liquid electrolyte solvent. Such an additive as trialkyl phosphite reacts with the lithium-aluminum-manganese alloy to produce an ionically conductive film on the lithium-aluminum-manganese alloy. This film inhibits the occurrence of a side reaction (decomposition reaction of the liquid electrolyte) between the liquid electrolyte and the lithium-aluminum-manganese alloy during charge-discharge cycles, resulting in obtaining excellent charge-discharge cycle performance characteristics.
In the present invention, the manganese content of the lithium-aluminum-manganese alloy is preferably in the range of 0.1-10 weight %, when given by that of an aluminum-manganese alloy into which lithium is subsequently inserted. If the manganese content falls outside the specified range, the ionically conductive film may not be produced in a satisfactory fashion.
Preferably, the lithium-aluminum-manganese alloy for use in the present invention may be in the form of a lithium-aluminum-manganese-vanadium or lithium-aluminum-manganese-chromium alloy. The use of such alloys permits formation of more effective films and thus results in obtaining particularly good charge-discharge cycle performance characteristics. The vanadium content of the lithium-aluminum-manganese-vanadium alloy is preferably in the range of 0.01-5 weight %, when given by that of an aluminum-manganese-vanadium alloy into which the lithium is subsequently inserted. The chromium content of the lithium-aluminum-manganese-chromium alloy is preferably in the range of 0.01-3 weight %, when given by that of an aluminum-manganese-chromium alloy into which lithium is subsequently inserted.
In the present invention, the additive is preferably incorporated in the amount of 0.1-20%, based on the total volume of the solvent and the additive. If its amount is below 0.1% by volume, the ionically conductive film may not be produced in a satisfactory fashion. On the other hand, if its amount exceeds 20% by volume, the film may be formed excessively thick to hinder the charge-discharge process. Accordingly, particularly good charge-discharge cycle performance characteristics are obtained when the amount by volume of the additive is 0.1-20% of the total volume of the solvent and the additive.
In the present invention, the positive electrode may be composed of any positive electrode material generally known to be useful for rechargeable lithium batteries. Examples of positive electrode materials include manganese dioxide, vanadium pentoxide, niobium oxide, lithium cobalt oxide, lithium nickel oxide, spinel manganese and the like. The improved charge-discharge cycle performance characteristics are obtained when a lithium-manganese complex oxide is used for the positive electrode material. Further improved charge-discharge cycle performance characteristics result when the lithium-manganese complex oxide is a complex oxide of lithium and manganese into which boron or boron compound is incorporated in the form of solid solution.
The lithium-manganese complex oxide containing boron or a boron compound in the form of solid solution is disclosed, for example, in Japanese Patent Laying-Open No. Hei 8-2769366 (1996). Specifically, a ratio of number of boron to manganese atoms (B/Mn) is 0.01-0.20. A mean valence number of manganese is at least 3.80. This complex oxide can be prepared by a method wherein a mixture of a boron, lithium and manganese compound, in a ratio of numbers of atoms (B:Li:Mn) of 0.01-0.20:0.1-2.0:1, is heat treated at a temperature of 150-430xc2x0 C., preferably of 250-430xc2x0 C., more preferably of 300-430xc2x0 C. If the temperature of heat treatment is below 150xc2x0 C., several problems arise including insufficient progress of reaction and insufficient moisture removal from MnO2. On the other hand, if the heat treatment temperature exceeds 430xc2x0 C., decomposition of MnO2 may be caused to occur to reduce a mean valence number of manganese to less than 3.80. As a result, the boron-containing lithium-manganese complex oxide during charge undergoes a change in electronic state to become unstable, resulting in the increased tendency to decompose and dissolve in the nonaqueous liquid electrolyte. The heat treatment is preferably performed in the air.
Examples of boron compounds include boron oxide (B2O3), boric acid (H3BO3), metaboric acid (HBO2), lithium metaborate (LiBO2) and lithium tetraborate (Li2B4O7). Examples of lithium compounds include lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium oxide (Li2O) and lithium nitrate (LiNO3). Examples of manganese compounds include manganese dioxide and manganese oxyhydroxide (MnOOH).
Examples of nonaqueous liquid electrolyte solutes found effective to give good charge-discharge cycle performances include lithium trifluoromethane sulfonimide, lithium pentafluoroethane sulfonimide and lithium trifluoromethane sulfonmethide, which will be later illustrated in the Examples.
Examples of nonaqueous liquid electrolyte solvents found effective to provide good charge-discharge cycle performances are mixed solvents containing at least one organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, xcex3-butyrolactone and sulfolane, and also containing at least one organic solvent selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyothane, 1,2-ethoxymethoxysthane, tetrahydrofuran, 1,3-dioxolane, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate. The liquid electrolytes containing such mixed solvents exhibit high ionic conductivity. When such mixed solvents are used in the preparation of the liquid electrolyte, a film having good ionic conductivity is formed on the negative electrode, resulting in the improved charge-discharge cycle performance characteristics.
The present invention utilizes a lithium-aluminum-manganese alloy for the negative electrode and the above-specified additive for incorporation in the liquid electrolyte. This prevents occurrence of a decomposition reaction of the liquid electrolyte on the negative electrode during charge, resulting in obtaining good charge-discharge cycle performance characteristics.